Hidden Illustrator layers show up in PDF files when reading in non-Adobe Reader: solution

Invisible layers will magically reappear when an Illustrator PDF is loaded in certain non-Adobe products (for example, the Mac PDF reader Preview). To avoid this issue, deselect “Preserve Illustrator Editing Capability” and “Create Acrobat Layers from top-level Layers” in the Save Adobe PDF dialog that pops up after choosing Save A Copy… from the File menu in Illustrator.